J-20... The New Generation Fighter II

[Engineer]

The J-20 has a few things different from the YF-23, first it has a F-22`s style fuselage, not the same design of the YF-23, it is longer than the F-22 and has a longer rear fuselage so it makes more drag, the canards are not arranged as on the J-10 and the wing is quit aft...

YF-23 is longer than the YF-22 yet have slightly higher top-speed. Given that both aircraft were intended to have have F-119 installed, their thrust would have been identical. Given the same thrust, the fact that YF-23 could fly faster implies less drag, and the fact that YF-23 being longer debunks your theory that longer fuselage results in more drag.

...the YF-23 has a similar fuselage to the PAKFA...

No it does not.

If the J-20 weighs 21000 or 22000kg at empty weigh and 40000 to 42000kg at max take off weight in order to have a 1:1 thrust to weight ratio will need at least engines of 19000 kg of thrust.

J-20 being slightly longer than F-22 and having similar cross sectional area simply could not be twice as heavy as the F-22.

 

[latenlazy]

question? why do you need variable geometry inlets?
answer because engines stall due to turbulent supersonic flow that the jet engine gets, therefore if a jet engine ingest supersonic flow it will lose power, and later stall.
at high speed without the use of a variable inlet the jet won`t get its max power, by the second law of newtwon then
Newton's laws of motion
Through Newton's second law, which states: The acceleration of a body is directly proportional to the net unbalanced force and inversely proportional to the body's mass, a relationship is established between Force (F), Mass (m) and acceleration (a). This is of course a wonderful relation and of immense usefulness.
F = m x a

if you jet loses thrust it loses force then you can not get to Mach 3 even having engines rate at 16000kg of power.

The MiG-25 has variable geometry inlet the F-22 does not

*Facepalm* Mach 3 is a measure of velocity (relative to viscosity). Force determines acceleration, or how fast your velocity increases. So long as the force is positive (oh man, Star Wars jokes) the aircraft will keep on accelerating. However, a plane does not fly in a vacuum (actually aerodynamic flight would be impossible in a vacuum). Air resistance increases as an object goes faster, so drag counteracts the force of an aircraft flying forward (it is a negative force). Furthermore, due to the laws of momentum an object that propels itself by pushing against another mass (jet and rocket engines in this case) cannot reach a velocity faster than how hard its pushing that mass. In the case of jet engines it means the specific velocity of the exhaust is the speed limit that engine can reach.

What does this mean? It means that so long as the engine can generate a positive force greater than drag, it will reach a top speed equal to the specific velocity of its exhaust. If the engine stalls then of course the plane won't be able to keep on accelerating, but that's not necessarily the case if the engine merely produces less thrust because it's not getting adequate airflow. That's why the F-22's top speed is likely above mach 2, because its engines can generate enough power even when its boundary layer diverters aren't diverting the entire boundary layer.

The wing is behind the main landing gear, in fact wing LERX is the only part of the wing ahead of the main landing gear, unless you claim the aerodynamic center and lift centers are ahead of the landing gear and positioned in the LERX, well you proposition is basily false.

The canard then is heavly loaded, amounting for a big chuck of total lif

compared to the J-10 the wing of the J-20 is far to aft to allow for an unstable configuration

What does the position of the landing gears have anything to do with the center of lift?

 

[MiG-29]

How are the ventral fins bad for stealth when they are canted? The size of the all moving vertical stabilizers combined with the ventral fins is definitely smaller than that of the massive vertical stabilizer on the F-22.

canting was a good solution to reduce radar signature, and it is possible the J-20`s vertical fins are 5% smaller in area than the F-22 but the J-20 uses a lot the dorsal vertical fins as pitch control thus increasing radar signature, the F-22 uses thrust vectoring as pitch control rather than aerodynamic surfaces

 

[MiG-29]

*Facepalm* Mach 3 is a measure of velocity (relative to viscosity). Force determines acceleration, or how fast your velocity increases. So long as the force is positive (oh man, Star Wars jokes) the aircraft will keep on accelerating. However, a plane does not fly in a vacuum (actually aerodynamic flight would be impossible in a vacuum). Air resistance increases as an object goes faster, so drag counteracts the force of an aircraft flying forward (it is a negative force). Furthermore, due to the laws of momentum an object that propels itself by pushing against another mass (jet and rocket engines in this case) cannot reach a velocity faster than how hard its pushing that mass. In the case of jet engines it means the specific velocity of the exhaust is the speed limit that engine can reach.

What does this mean? It means that so long as the engine can generate a positive force greater than drag, it will reach a top speed equal to the specific velocity of its exhaust. If the engine stalls then of course the plane won't be able to keep on accelerating, but that's not necessarily the case if the engine merely produces less thrust because it's not getting adequate airflow. That's why the F-22's top speed is likely above mach 2, because its engines can generate enough power even when its boundary layer diverters aren't diverting the entire boundary layer.


What does the position of the landing gears have anything to do with the center of lift?

Length 62 ft / 18.90 m
Height 16.7 ft / 5.09 m
Wingspan 44.5 ft / 13.56 m
Wing area 840 sq ft / 78.04 sq m
Horizontal tail span 29 ft / 8.84 m
Weight empty 43,340 lb /19,700 kg
Maximum take-off weight 83,500 lb / 38,000 kg
Internal fuel
with two external wing tanks 18,000 lb / 8,200 kg
26,000 lb / 11,900 kg
Speed Mach 2 class
Range* > 1,600 n. mi
Power plant Two F119-PW-100 turbofan engines with two-dimensional thrust vectoring nozzles
Engine thrust 35,000 lb / 15,876 kg

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where does it say in the lockheed martin webpage the F-22 achieves Mach 2.3?

 

[Engineer]

The wing is behind the main landing gear, in fact wing LERX is the only part of the wing ahead of the main landing gear, unless you claim the aerodynamic center and lift centers are ahead of the landing gear and positioned in the LERX, well you proposition is basily false.

My proposition that you have no information to conclude that J-20 is positively stable in the longitudinal axis is entirely correct, since you in fact do not have information regarding aerodynamic center of the aircraft. Also, you are still confuse between the aerodynamic center of the wings with the aerodynamics of the aircraft. The wings being placed aft of the landing gears does not mean the aerodynamic center of the aircraft is also behind the landing gears, especially when there are two huge canards in the front.

The canard then is heavly loaded, amounting for a big chuck of total lift.

Yes, so?

compared to the J-10 the wing of the J-20 is far to aft to allow for an unstable configuration

Without knowledge in either the position of the aerodynamic center of the aircraft or the center-of-gravity, you cannot make an inference for such a conclusion.

 

[latenlazy]

canting was a good solution to reduce radar signature, and it is possible the J-20`s vertical fins are 5% smaller in area than the F-22 but the J-20 uses a lot the dorsal vertical fins as pitch control thus increasing radar signature, the F-22 uses thrust vectoring as pitch control rather than aerodynamic surfaces

To quote myself:
I could take your speculative approach and come up with a completely different conclusion by focusing on different things. Why would they make the dorsal fins smaller to the point of needing ventral fins when they could just solve lateral stability problems by making the dorsal fins bigger? It must be because 1) because adding the ventral fins might be better low signature design than making the dorsal fins bigger, or 2) because the flight control system is incomplete and the ventral fins are added for extra security, and will be removed afterwards. I subscribe to neither simply because I don't know anything about how the overall shape of the airframe affects aerodynamics and stealth. The main point is your reliance on these kind of piecemeal arguments show that neither do you.

The J-20 may be using its vertical stabilizers for pitch control, but it's also going to have TVC installed on it. Another indication that the ventral fins are only temporary?

Length 62 ft / 18.90 m
Height 16.7 ft / 5.09 m
Wingspan 44.5 ft / 13.56 m
Wing area 840 sq ft / 78.04 sq m
Horizontal tail span 29 ft / 8.84 m
Weight empty 43,340 lb /19,700 kg
Maximum take-off weight 83,500 lb / 38,000 kg
Internal fuel
with two external wing tanks 18,000 lb / 8,200 kg
26,000 lb / 11,900 kg
Speed Mach 2 class
Range* > 1,600 n. mi
Power plant Two F119-PW-100 turbofan engines with two-dimensional thrust vectoring nozzles
Engine thrust 35,000 lb / 15,876 kg

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where does it say in the lockheed martin webpage the F-22 achieves Mach 2.3?

Because its engines are sufficiently powerful enough to not need optimal airflow (aka, despite losing some boundary layer control) in order to generate a positive forward force (though not necessarily maximum thrust), and because its engines have a high specific velocity. If you didn't notice, that's an elaboration of the reason I had already given in the post you replied. To quote myself again:

hat does this mean? It means that so long as the engine can generate a positive force greater than drag, it will reach a top speed equal to the specific velocity of its exhaust. If the engine stalls then of course the plane won't be able to keep on accelerating, but that's not necessarily the case if the engine merely produces less thrust because it's not getting adequate airflow. That's why the F-22's top speed is likely above mach 2, because its engines can generate enough power even when its boundary layer diverters aren't diverting the entire boundary layer.

Without knowledge in either the position of the aerodynamic center of the aircraft or the center-of-gravity, you cannot make an inference for such a conclusion.

On another point, wouldn't any assessment of center of lift be further complicated by the fact that the plane has a degree of wing-body blending?

 

[Engineer]

canting was a good solution to reduce radar signature, and it is possible the J-20`s vertical fins are 5% smaller in area than the F-22 but the J-20 uses a lot the dorsal vertical fins as pitch control thus increasing radar signature, the F-22 uses thrust vectoring as pitch control rather than aerodynamic surfaces

Radar signature is only a consideration while the aircraft is at cruise, not while the plane is taking off.

 

[MiG-29]

Radar signature is only a consideration while the aircraft is at cruise, not while the plane is taking off.

without thrust vectoring how do you control the jet? V tails are used o the J-20 as pitch control

 

[latenlazy]

without thrust vectoring how do you control the jet? V tails are used o the J-20 as pitch control

The J-20 will have thrust vectoring. We know that much already.

 

[MiG-29]

*Facepalm* Mach 3 is a measure of velocity (relative to viscosity). Force determines acceleration, or how fast your velocity increases. So long as the force is positive (oh man, Star Wars jokes) the aircraft will keep on accelerating. However, a plane does not fly in a vacuum (actually aerodynamic flight would be impossible in a vacuum). Air resistance increases as an object goes faster, so drag counteracts the force of an aircraft flying forward (it is a negative force). Furthermore, due to the laws of momentum an object that propels itself by pushing against another mass (jet and rocket engines in this case) cannot reach a velocity faster than how hard its pushing that mass. In the case of jet engines it means the specific velocity of the exhaust is the speed limit that engine can reach.

What does this mean? It means that so long as the engine can generate a positive force greater than drag, it will reach a top speed equal to the specific velocity of its exhaust. If the engine stalls then of course the plane won't be able to keep on accelerating, but that's not necessarily the case if the engine merely produces less thrust because it's not getting adequate airflow. That's why the F-22's top speed is likely above mach 2, because its engines can generate enough power even when its boundary layer diverters aren't diverting the entire boundary layer.


What does the position of the landing gears have anything to do with the center of lift?

the position of the landing gear is a way to know the most likely position of the center of gravity, no main landing gear will be ahead of the center of gravity